From: Dale Eason 
To: Russell Jocoy , atm{at}shore.net
Reply-To: Dale Eason 



-+- Russell Jocoy  wrote:
>    Well bust my britches, ., I do think that on a
> long focus truss type Dob
> that there may be some
> fluxuation in focus.

I'm not sure what you mean by that.  But according to Suiter in "Star
Testing Astronomical Telescopes". Table 5-1a Page 80 Defocus distances
for different focal ratios.  Here he is saying how much an eyepiece must be
moved to defocus it by .5 a wave length aberration.  Much more than you
would normally do with one eyepiece while looking at far away objects.

F4 .0014 inch,  f8 .005 inch, f12 .012 inch F22 .042 inch.

As you can see the movement is very tiny. Your eye does not have to move
much to follow the eyepiece. Assuming it was already in focus on another
object to start with.

I think this shows however that if your optics shift a small amount as you
move the scope around you will need to refocus an F4 much more often than
an f12.  So it is just the opposite of what you assumed.

Dale Eason

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